The present invention relates to access to resources of a radiocommunication system. It relates more particularly to access to resources by mobile terminals that are distant from the base stations of the radiocommunication system.
In such radiocommunication systems, such as for example in the UMTS (“Universal Mobile Telecommunication System”) system, mobile terminals are devised to communicate with the network by way of at least one base station.
Each base station possesses a zone of radio coverage inside which it can exchange radio signals with terminals. The size of a coverage zone depends on several parameters, such as for example the transmit power of the base station concerned. Specifically, below a certain transmit power, the signals transmitted to a radio terminal will be received too weakly for the latter to be capable of decoding them. The same is true for the uplink: the maximum transmit power of the terminals may represent a limiting factor in respect of the size of the zone where the communication service is possible.
However, other constraints exist in such systems. Some of them may prove to be particularly limiting, so much so that they impose the strictest delimitation on the zones of coverage.
In the UMTS system for example, the tightest constraint is imposed by the access procedure. Each base station in fact possesses reception windows inside which it can detect access requests sent by terminals in corresponding slots of send instants. An access request received by a base station between two successive reception windows will not be detected correctly by the base station.
An access request is characterized initially by the transmission of a preamble of 4096 chips (elementary units for the transmission of binary information) by a mobile terminal. Now, a reception window for such a preamble, corresponding to a “slot” of the PRACH (“Physical Random Access CHannel”) random access channel, possesses 5120 chips. There are therefore 1024 (=5120-4096) possible positions for the reception of a preamble. The base station thus performs correlation calculations for the entire set of these 1024 positions, so as to detect the possible presence of an access request inside a reception window.
Given the foregoing, it can be proven that the maximum distance at which a mobile terminal may lie with respect to a base station, in order that the preamble that it sends be received by the base station in the corresponding reception window, is only 40 kilometres. Beyond this limit, the access request made by the mobile terminal will probably not be received in a send window by the base station concerned, and this will prevent the access procedure from continuing: the mobile terminal will therefore not be allocated any communication resource.
In certain cases however, it would be particularly beneficial to be able to extend the zone of coverage of a base station to access for setting up a communication, as much as the other constraints limiting the zone of coverage, in particular during communication, are much less restrictive. An extension of the coverage may turn out to be extremely useful in particular in maritime zones far from coasts, for example to set up an emergency call.
An object of the present invention is to alleviate the limitations of the current technique, by allowing an increase in the size of the zone of coverage of base stations to access.
Another object of the present invention is to allow access to the network of mobile terminals that are distant from the base stations of the radiocommunication network, in certain cases where this turns out to be necessary, for example to set up emergency calls.